Optically bound particle structures in evanescent wave traps

We present a study of the manipulation of microparticles and the formation of optically bound structures of particles in evanescent wave traps. Two trapping geometries are considered: the first is a surface trap where the evanescent field above a glass prism is formed by the interference of a number of laser beams incident on the prism-water interface; the second uses the evanescent field surrounding a biconical tapered optical fiber that has been stretched to produce a waist of submicron diameter. In the surface trap we observe optical binding of microparticles in to one-dimensional chain structures. In the tapered optical fiber trap we demonstrate both particle transport for long distances along the fiber, and the formation of stable arrays of particles. In both experiments we use video microscopy to track the particle locations and make quantitative measures of the particle dynamics. The experimental studies of particle structures are complemented by light scattering calculations based on Mie theory to infer how the geometries of the observed particle structures are controlled by the underlying incident and scattered optical fields.